The prime minister has said she wants to "strengthen the bonds that unite" Wales and the UK, as she prepares to mark one year to go until Brexit.
Theresa May visits Vale of Glamorgan firms on Thursday on a UK-wide tour.
With the UK government negotiating with Welsh ministers over assembly powers after Brexit, she will say she is "committed" to devolution.
But Wales' First Minister Carwyn Jones warned her Brexit plan would "do serious damage to our economy".
The Welsh Government backs the UK remaining in the EU single market and its customs union, which means member countries club together to apply the same import taxes on goods coming from outside the union.
In a speech in March, Mrs May reiterated her plan to leave both the single market and the customs union as she called for a free trade agreement covering most sectors of the economy.
She warned that "no-one will get everything they want" out of the Brexit negotiations leading Mr Jones to criticise the speech for having "too many vague aspirations and scant detail".
At the time, the first minister said the prime minister failed to "spell out" how she intends to achieve "frictionless" trade with the EU "outside a customs union".
In his latest statement, Mr Jones said that with the UK set to leave the EU at 23:00 BST on 29 March 2019 "the people of Wales still have no idea about the post-Brexit deal the prime minister wants with Brussels".
He added: "The clock is ticking. Businesses and the public sector need to be able to plan for this huge change but the lack of clarity from the UK government is making this all but impossible.
"I am not questioning Brexit - the UK is leaving the EU. But we deserve to know what the plan is.
"We will continue to fight for a sensible Brexit that protects Wales' economy and jobs."
Mrs May will visit all four countries of the UK on Thursday, including Ayrshire in Scotland, Newcastle, Belfast, Barry in Vale of Glamorgan before finishing in west London.
Speaking ahead of the visits, the prime minister said: "I am determined that as we leave the EU, and in the years ahead, we will strengthen the bonds that unite us, because ours is the world's most successful union.
"The UK contains four proud and historic nations, but together we amount to so much more than the sum of our parts and our Union is an enormous force for good."
Referring to an ongoing row with the Welsh Government over what ministers in Cardiff have dubbed a "power-grab", the prime minister said "each of the devolved nations will see an increase in their decision-making powers" after Brexit.
"Make no mistake, this government is absolutely committed to the devolution settlements as we have demonstrated beyond question with landmark pieces of legislation over the last few years," she said.
"But as the prime minister of the United Kingdom, I have an absolute responsibility to protect the integrity of the United Kingdom as a whole.
"That means ensuring that no new barriers are created within our common domestic market and that the UK is able to meet its international obligations in the future.
"No prime minister could leave these things to chance, because they are absolutely crucial to our success as a country in the future."
Last week, assembly members in Cardiff Bay passed a law - known as the Continuity Bill - as an insurance policy against the UK government's so-called Withdrawal Bill.
Plaid Cymru's Brexit spokesman Steffan Lewis said: "Our own parliament, the National Assembly has made it very clear that we will resort to legislating to protect ourselves from Westminster's power grab, with the support of every party represented there.
"The Continuity Bill will simply ensure that all powers in devolved areas will remain devolved after Brexit.
"Theresa May needs to use her visit to Wales to reassure our citizens that the Continuity Bill will be respected, and that it won't be challenged in the courts by Westminster.
"If the prime minister believes there is no such power-grab in the first place, she should have no concerns about legislation that maintains our existing powers."